Two current examples of probable correspondence of independently discovered systems suggest that the present inventory of separate polymorphic systems expressed selectively or exclusively by lymphocytes may be at least approaching the actual total number demonstrable by immunogenetic methods. Thus (1)\the system defined by hybridoma clone 7-4.2 may correspond to Ly-15.2; and (2)\the system defined by Ly-m20.2 may represent the alternative allele of Ly-17.1. Likewise, Ly-m11 probably corresponds to beta-2M. Monoclonal antibody precipitates radiolabeled cytoplasmic and mature plasma-membrane Ly-15.2 products, and we have data on structure and synthesis. Serological evidence of further polymorphism in the exclusive B-cell system Lyb-2 is reported, and a corresponding structural study of Lyb-2 polymorphism is under way. Further serological definition of Ly-5 is emphasized, because this system is exclusively represented throughout the hematopoietic compartment of development by expression of multiple molecular isoforms, each typifying a cell lineage. A potentially critical serological observation, now confirmed, is that conventional Ly-5 antisera appear to recognize nonreciprocal Ly-5 allotypes expressed by the ST/bJ mouse strain, a finding which may bear on the complexity or operation of the Ly-5 locus of chromosome 1 and which is being studied by the derivation of a second Ly-5 congenic strain and production of further Ly-5 monoclonal antibodies from new immunizations. From the results of assays of immune function in vitro, which imply that Ly-5 is involved in the reception of macrophage message by B cells, it is proposed that the common function of Ly-5 on all hematopoietic cell sets may be the reception or mediation of regulatory intercellular signals. In the Lyt-1 system, the conclusion from studies of synthesis and allotypic differences in peptide mapping is that the Lyt-1 locus of chromosome 19 is the site of the Lyt-1 protein structural gene. (CS)